Add script to validate files are valid DocBook.

Adds a script called tools/validate.py that will check all XML
files against the DocBook 5 RELAX NG schema.

Does not check pom.xml files or files inside of directories named
"target".

Cherry-picked from https://review.openstack.org/18170

Change-Id: I9614f1b10ac2da608a245c7644309a5be38c8995
Merge "Add script to validate files are valid DocBook." into stable/folsom